U.S. patent number 6,313,428 [Application Number 09/416,574] was granted by the patent office on 2001-11-06 for apparatus and method for reducing space charge of ion beams and wafer charging.
This patent grant is currently assigned to Advanced Ion Beam Technology, Inc.. Invention is credited to Jin-Liang Chen, Linuan Chen.
United States Patent |
6,313,428 |
Chen , et al. |
November 6, 2001 |
Apparatus and method for reducing space charge of ion beams and
wafer charging
Abstract
An apparatus for ion beam neutralization is disclosed in this
invention. The apparatus is a plasma flood source with an arc
discharge chamber enclosed in a source housing with sufficient
cooling so that the housing temperature is near room temperature.
Arc discharge between a filament and the arc chamber ionizes the
bleeding gas atoms or molecules in the arc chamber and produces
plasma. The low energy electrons together with ions in the plasma
drift out of the arc chamber and neutralize the passing ion beam.
The sufficiently cooled source housing prevents radiation to the
processed wafers, reduces metal particle concentration in the
plasma and therefore metal contamination on the wafers, and keeps
beamline pressure low while more electrons are extracted from the
flood source through the apertures with larger area.
Inventors: |
Chen; Jin-Liang (Cupertino,
CA), Chen; Linuan (San Jose, CA) |
Assignee: |
Advanced Ion Beam Technology,
Inc. (Sunnyvale, CA)
|
Family
ID: |
23650498 |
Appl.
No.: |
09/416,574 |
Filed: |
October 12, 1999 |
Current U.S.
Class: |
219/121.43;
219/121.33; 219/121.48; 219/121.49; 250/492.1 |
Current CPC
Class: |
H01J
37/026 (20130101); H01J 2237/0041 (20130101); H01J
2237/31701 (20130101) |
Current International
Class: |
H01J
37/02 (20060101); B23K 010/00 () |
Field of
Search: |
;219/121.43,121.48,121.49,121.59,121.21,121.33
;250/492.1,492.2,492.21,251 ;204/298.12 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Paschall; Mark
Attorney, Agent or Firm: Lin; Bo-In
Claims
We claim:
1. A plasma flood source comprising:
a source housing;
a gas bleeding inlet for introducing gaseous particles into said
source housing;
a filament disposed in said source housing for transmitting a
filament current and emitting thermal electrons therefrom for
colliding with said gaseous particles to form a gas plasma with
ionized gaseous particles; and
said source housing comprising a plurality of hollow walls each
having a coolant passway for passing coolant therethrough for
reducing a temperature of said source housing for depositing a
plurality of metallic ions emitted from said filament thereon.
2. The plasma flood source of claim 1 wherein:
said source housing further comprising holes disposed on one of
said hollow walls near said filament constituting plasma extraction
apertures for extracting said gas plasma and thermal electrons
through said extraction aperture.
3. The plasma flood source of claim 1 wherein:
said hollow walls of said source housing comprising aluminum hollow
walls.
4. The plasma flood source of claim 1 further comprising:
a coolant inlet for introducing coolant into said coolant passways
and a coolant outlet for releasing coolant out for said coolant
passways.
5. The plasma flood source of claim 1 further comprising:
said power supply connected to said filament for generating said
filament current for transmitting through said filament.
6. The plasma flood source of claim 5 wherein:
said power supply connected to said filament further providing a
negative voltage bias to said filament relative to said hollow
walls.
7. The plasma flood source of claim 1 wherein:
said filament comprising a tungsten filament.
8. An ion implanting apparatus for introducing an ion beam to an
implant target comprising:
a plasma flood source having source housing comprising a plurality
of hollow walls each having a coolant passway for passing coolant
therethrough for reducing a temperature of said source housing for
depositing a plurality of metallic ions emitted from said filament
thereon.
9. A ion implanting apparatus for introducing an ion beam to an
implant target comprising:
a plasma flood source having source housing comprising a plurality
of hollow walls each having a coolant passway for passing coolant
therethrough for reducing a temperature of said flood source.
10. The ion implanting apparatus of claim 9 wherein:
said plasma flood source further comprising a gas bleeding inlet
for introducing gaseous particles into said source housing.
11. The ion implanting apparatus of claim 10 wherein:
said plasma flood source further comprising a filament disposed in
said source housing for transmitting a filament current and
emitting thermal electrons therefrom for colliding with said
gaseous particles to form a gas plasma with ionized gaseous
particles.
12. The ion implanting apparatus of claim 11 wherein:
said source housing further comprising holes formed in one of said
hollow walls disposed near said filament constituting plasma
extraction apertures for extracting said gas plasma and thermal
electrons through said extraction aperture.
13. The ion implanting apparatus of claim 9 wherein:
said hollow walls of said source housing comprising aluminum hollow
walls.
14. The ion implanting apparatus of claim 9 wherein:
said plasma flood source further comprising a coolant inlet for
introducing coolant into said coolant passways and a coolant outlet
for releasing coolant out for said coolant passways.
15. The ion implanting apparatus of claim 11 wherein:
said plasma flood source further comprising a power supply
connected to said filament for generating said filament current for
transmitting through said filament.
16. The ion implanting apparatus of claim 15 wherein:
said power supply connected to said filament further providing a
negative voltage bias to said filament relative to said hollow
walls.
17. The ion implanting apparatus of claim 11 wherein:
said filament comprising a tungsten filament.
18. The ion implanting apparatus of claim 12 wherein:
said plasma extraction apertures disposed near said ion beam for
extracting and introducing said gas plasma and thermal electrons to
said ion beam through said extraction apertures.
19. The ion implanting apparatus of claim 18 wherein:
said plasma flood source is disposed at a particular position
relative to said ion beam whereby said gas plasma and thermal
electrons are introduced to said ion beam at a direction
perpendicular to a beamline of said ion beam through said
extraction apertures.
20. A method of introducing electrons into an ion beam for
implanting a target wafer comprising:
a) forming a source housing by employing a plurality of hollow
walls each having a coolant passway for passing coolant
therethrough for reducing a temperature of said source housing near
a room temperature.
21. The method of claim 20 further comprising:
b) introducing gaseous particles into said source housing through a
gas bleeding inlet.
22. The method of claim 21 further comprising:
c) transmitting a filament current through a filament disposed in
said source housing for emitting thermal electrons therefrom for
colliding with said gaseous particles to form a gas plasma with
ionized gaseous particles.
23. The method claim 22 further comprising:
d) extracting said gas plasma and thermal electrons through holes
formed as plasma extraction apertures in one of said hollow walls
near said filament.
24. The method of claim 20 wherein:
said step a) of forming said source housing with said hollow walls
comprising a step a1) of forming said hollow walls as aluminum
hollow walls.
25. The method of claim 20 wherein:
said step a) of forming said source housing with said hollow walls
further comprising a step a2) of providing a coolant inlet for
introducing coolant into said coolant passways and a step a3) of
providing a coolant outlet for releasing coolant out for said
coolant passways.
26. The method of claim 22 wherein:
said step c) of transmitting a filament current through a filament
further comprising a step c1) of connecting a power supply to said
filament for generating said filament current for transmitting
through said filament.
27. The method of claim 26 wherein:
said step c1) of connecting said power supply to said filament
further comprising a step of c2) providing a negative voltage bias
to said filament relative to said hollow walls.
28. The method of claim 22 wherein:
said step of transmitting a filament current through a filament
further comprising a step c3) of transmitting said filament current
through a tungsten filament in said source housing.
29. The method of claim 23 wherein:
said step d) of extracting said gas plasma and thermal electrons
through said plasma extraction apertures further comprising a step
of d1) extracting and introducing said gas plasma and thermal
electrons to said ion beam through said extraction apertures.
30. The method of claim 29 wherein:
said step d1) of extracting and introducing said gas plasma and
thermal electrons to said ion beam through said extraction
apertures plasma flood source further comprising a step of d2)
introducing said gas plasma and thermal electrons to said ion beam
at a direction perpendicular to a beamline of said ion beam through
said extraction apertures.
31. The method of claim 20 wherein:
said step a) of forming a source housing by employing a plurality
of hollow walls each having a coolant passway for passing coolant
therethrough further comprising a step a1) of passing coolant for
reducing a temperature of said plasma flood source from near 1200K
to near 300K.
32. The method of claim 31 wherein:
said step a) of forming a source housing by employing a plurality
of hollow walls each having a coolant passway for passing coolant
therethrough further comprising a step a2) of passing coolant for
reducing a gas pressure inside said source housing by a factor of 4
substantially according to a formula of p=nkT, where n is a gas
density, p a pressure, T a temperature, and k a Boltzman
constant.
33. The method of claim 28 wherein:
said step a) of forming a source housing by employing a plurality
of hollow walls each having a coolant passway for passing coolant
therethrough further comprising a step a2) of passing coolant for
reducing a gas pressure inside said source housing by a factor of 4
substantially according to a formula of p=nkT, where n is a gas
density, p a pressure, T a temperature, and k a Boltzman constant;
and
said step d1) of extracting and introducing said gas plasma and
thermal electrons to said ion beam through said extraction
apertures plasma flood source further comprising a step of d3) of
opening each of said extraction apertures with an open area
corresponding a said source housing pressure for introducing said
electrons to said ion beam.
34. A method of reducing metal contamination by a plasma flood
source to an implanting ion beam comprising:
a) providing a cooling means for reducing a temperature of said
plasma flood source to reduce metal ions introduced to said
implanting ion beam emitted from said plasma flood source.
35. The method of claim 34 further comprising a step of:
forming said plasma flood source with aluminum and tungsten wherein
said step of reducing a temperature of said plasma flood source is
a step of reducing tungsten and aluminum ions emitted from said
plasma flood source.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the ion implantation systems
and methods employed in the fabrication process for manufacturing
semiconductor devices. More particularly, this invention relates to
apparatus for effecting the ion beam neutralization for ion
implantation and, more particularly, relates to an apparatus for
neutralizing a positively charged ion beam by providing low energy
electrons from a sufficiently cooled plasma flood source.
2. Description of the Prior Art
Electrical charge buildup on a wafer surface when ion beams are
applied for implantation is still a technical difficulty faced by
the semiconductor industries. Ion implantation has been used as a
standard way to dope impurities, such as boron, phosphorus, and
arsenic, into semiconductor materials for more than two decades.
Its property of controllable doping direction makes it
irreplaceable by other doping techniques in manufacturing
sub-micron integrated circuits. An ion implanter used for ion
implantation purpose usually includes an ion source, ion beam
transport optics, and a process chamber where ion implantation
occurs. An electron or plasma flood source is always an important
component of an implanter. Not only can it help to reduce charge
build-up on target wafers, but also it can help to increase ion
beam transportation efficiency to the target wafers, especially for
low-energy ion beams.
It is well known in the art of integrated circuit (IC) manufacture
that a charged ion beam will produce a buildup of charge on the
surface of the semiconductor target. This charge may not be removed
from the surface of insulating and semi-conductive wafer material.
In this situation, positive charge builds up on the surface of the
material since most ion beams are positively charged. Such charge
may interfere automatic wafer handling due to sticking, may break
through layers of micro-circuitry, and may affect implant
uniformity due to charged portions of the wafer surface deflecting
the ion beam. The presence of such surface charges therefore is
believed to reduce yields in the production of integrated
circuits.
One effective way to reduce positive charge build-up on wafer
surface is to supply similar amount of negative charges, e.g.
electrons, on the wafer surface. Electron or plasma flood sources,
or alternatively called electron or plasma shower sources, are
usually used to supply these electrons and introduce them onto the
wafer as the ion beam strikes the wafer surface. As the requirement
of implantation expands towards low energy ions with large current,
e.g. for shallow junction formation, electron and plasma floods
were also used to reduce space charge blow-up and to enhance the
transportation of low energy ion beams with large currents.
To achieve the dual purposes of reducing wafer charging and space
charge blow-up, electrons are directed towards the passing ion beam
instead towards the target wafers, since the direct application of
electrons to the wafers can produce contamination from the filament
of the electron source. The direct radiation from the filament can
cause wafer heating and non-uniformity across the wafer during
implantation. The primary electrons from the flood source with high
energies, e.g. 70 eV, can also induce high negative potential on
wafer surface and may damage the integrated circuits.
For above reasons, the electrons are introduced in the beam
generally transverse to the direction of beam projection to produce
a neutralization of the beam. The individual ions in the beam are
not neutralized since the combination cross-section of an ion and
an electron is small. A neutralization of a beam means an effective
neutralization of charge within the volume of the beam. The
electrons in the neighborhood of the ion beam are attracted by the
positive beam potential and travel together with the ion beam.
However, with this approach the efficiency of entrapment of
electrons within the beam may be low due to the high velocity of
the primary electrons emitting from the filament and to the low
capture cross-section of the electrons by the beam. Generating
electrons with low energies would increase the probability of
entrapment.
There are several ways to produce low energy electrons. One is to
generate these electrons outside the primary electron source. In
this case, a dummy target, usually made of aluminum or graphite, is
located on the other side of the ion beam opposite to the electron
source. The primary electrons with energies about 70 eV are
extracted and strike the target, generating large amount of
secondary electrons. Most of these secondary electrons have much
lower energies, e.g. less than 10 eV.sup.1, and easier to be
trapped with the ion beam. This type of electron flood source has
been described in many patents, e.g., D. A. Robertson et al.,
"Apparatus for enhanced neutralization of positively charged ion
beam", U.S. Pat. No. 4,463,255, 1984, V. M. Benveniste, "Ion beam
neutralization means generating diffuse secondary emission electron
shower", U.S. Pat. No. 5,164,599, 1992, and J. D. Bernstein et al.,
"Biased and serrated extension tube for ion implanter electron
shower", U.S. Pat. No. 5,903,009, 1999. There are several technical
difficulties in applying this electron source for ion implantation.
Specifically, the primary electrons, through elastic collisions
with other electrons, may drift to wafers and produce high electric
fields on wafer surfaces thus induces micro-circuitry damage. For
effective neutralization, the dummy target has to be close to the
ion beam. The patented electron source increases the opportunity
for the ion beam to strike the dummy target during beam tuning and
create metal particles from the dummy target surface. These
particles will drift to the processed wafers and induce metal
contamination on wafer surfaces.
The second way to generate low energy electrons is to create them
inside the primary electron source. An ionizable gas is introduced
into the source. The primary electrons impact the gas atoms, knock
off one or more electrons from each atom, and generate plasma in
which positive charges (atomic ions) and negative charges
(electrons) are almost equal. The electron energies inside the
plasma depend on the plasma temperature, and are usually less than
5 eV with narrow distribution. These low energy electrons together
with ions, or the plasma, would drift out of the flood source and
reach the ion beam, as illustrated in FIG. 1 below in the section
of "Detail Description of Preferred Embodiment", therefore this
type of source is called plasma flood source. Plasma flood sources
have been described in many patents, e.g., H. Ito et al., "Plasma
flood system for the reduction of charging of wafers during ion
implantation", U.S. Pat. No. 5,399,871, 1995. The possibility to
trap the electrons in the ion beam is inverse proportional to the
electron velocities. Since the fast moving electrons are dragged by
the slowly moving heavy ions from the flood source, the electrons
have more time to stay around the ion beam and easier to the
trapped in the ion beam. When more ions in the ion beam are
neutralized after trapping greater number of electrons, its net
beam potential decreases, thus reduces the number of trapped
electrons. Therefore, this type of electron flood source can
self-regulate the amount of trapped electrons to avoid over or
under compensating the positive beam current. Since the electrons
from the source have low energy, the wafer damage caused by
high-energy electrons can be avoided. There is no dummy target
required, and the flood source can be placed far away from the ion
beam, therefore there is minimum chance that the ion beam will
strike any metal surface near the wafers except the process
chamber. However, the discharge inside the flood source can still
generate metallic particles such as tungsten, aluminum, or
molybdenum, depending the building materials of the flood source
housing. These metallic particles can drift out of the source and
deposition onto the wafer surfaces.
Reducing metal contamination is important for increasing the yield
of integrated circuit manufacturing. A person of ordinary skill in
the art still faces the demand of providing an improved flood
source that induces much less metal contamination while keeps most
advantages of other conventional flood sources.
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide an
apparatus for producing sufficient neutralization of a positively
charged beam for better beam transportation, especially for
low-energy ion beams.
It is a further object of the present invention to provide an
apparatus for producing sufficient low-energy electrons on
processed wafers to reduce wafer charging.
It is a further object of the present invention to provide an
apparatus that produces minimum amount of metal particles onto
processed wafers.
It is an additional object of the present invention to provide an
apparatus that can accomplish the above goals with minimum
radiation on processed wafers.
To reduce space charge blow-up and charge build-up on wafers,
electron floods are usually applied to neutralize the ion beams.
Thermal electrons are extracted from the electron flood sources
into the ion beams and travel together toward the wafers. The
conventional electron floods have secondary electron type and
plasma flood type. They can increase beam transmission from the ion
source to the wafers and reduce wafer charging at the same time.
However, almost all electron flood sources in the market create
metal contamination and wafer heating. An improved newly designed
plasma flood source is disclosed in this invention that can
overcome these problems by sufficient cooling of the flood body. By
significantly reducing the temperature of flood body, the bleeding
gas pressure inside the flood body is also reduced. The flood
extraction aperture area can be increased to provide more
neutralizing electrons without affecting gas pressure inside the
beam-line. The new plasma flood can provide much more efficient
beam neutralization, especial at low ion energy, with minimum metal
contamination and wafer heating.
The present invention therefore has the advantages that it provides
a new plasma flood source for better space charge neutralization,
lower wafer charging, and less metal contamination. The new plasma
flood source with better cooling will enable those of ordinary
skill in the art to overcome the difficulties, encountered in the
prior art.
Specifically, the present invention provides a new plasma flood
source with the whole source wall cooled to room temperature. The
low temperature environment allows the source wall to be made of
aluminum only and eliminates the usage of other metals for the
source components, such as molybdenum and copper, except for the
tungsten filament. The new plasma flood source confines the
possible metal contamination at the wafers to only aluminum and
tungsten from the electron flood source.
Another advantage of the present invention is that it provides a
new plasma flood source with cooled source wall to reduce aluminum
vaporization and increases possibility of tungsten and aluminum
deposition onto the cooled source wall. The partial pressure of
tungsten and aluminum vapor inside the flood source housing can be
greatly reduced. The metal contamination contributed from the flood
sources usually comes from the metal vapor drifting out of the
source from the flood extraction aperture. When the tungsten and
aluminum vapor partial pressure is reduced inside the source, the
metal contamination is also reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram of a plasma flood source of
this invention; and
FIG. 2a is a three-dimensional mechanical design diagram of the
plasma flood source of this invention, with several plasma
extraction apertures in the front source surface; and
FIG. 2b is a three-dimensional mechanical design diagram of the
plasma flood source of this invention, without the front source
surface, so that the filament and the water-cooling channel can be
seen.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention introduces a new plasma flood source that has
a cold source housing. FIG. 1 is a functional block diagram for
showing the plasma flood source 10 of this invention. The plasma
flood source 10 includes a source housing 20 with a hollow wall, an
arc plasma chamber 30 in the upper part of the source housing, and
a filament 35 inside the arc plasma chamber 30. Cold water flows
into the water passway 20' formed in the hollow wall of the source
housing 20 from the water inlet 50 and comes out from the water
outlet 55 to keep the source housing near room temperature. An
ionizable gas is introduced into the arc chamber 30 through the gas
inlet 40. When a large current is running through the filament 35
by the filament current supply 60, thermal electrons emit from the
filament 35 into the arc chamber 30. When the filament 35 is biased
at a negative potential by the filament-biasing voltage 65, the
emitting electrons have sufficient kinetic energies to ionize the
gas molecules inside the arc chamber 30 and create arc plasma.
Electrons and ions in the plasma drift out of the extraction
apertures 25 toward the ion beam 80 and form plasma flood 70. The
low energy electrons in the flood are trapped in the positive ion
beam 80 and neutralize the beam 80.
FIG. 2 shows a mechanical design of a plasma flood source of this
invention. The hollow source housing and filament are clearly shown
in FIG. 2b. The cooling water running through the hollow source
housing cools the walls of the arc chamber efficiently.
The flood source external surfaces facing the wafers are near room
temperature because of the sufficient water-cooling. The filament
is installed at a position so that there is no filament exposure to
the wafers through those apertures. Therefore there is not direct
radiation from the filament 35 to irradiate on the target wafer
thus causes a side effect of wafer heating.
A conventional plasma flood source without sufficient water-cooling
can have temperatures as high as 1200K caused by arc discharge from
the filament to the arc chamber wall. Metallic particles from the
filament and the chamber wall can hardly stick to the hot wall.
They will eventually leak out, together with the plasma flood, and
cause metal contamination on the processed wafers. In the plasma
flood source of this invention, the source housing is cooled to
near 300K, a factor of 4 lower than a conventional source. The
metallic particles inside the arc chamber have much higher
possibility to deposit on the cool chamber wall. The coolant flows
through the water channel 20' formed in the hollow wall of the
source housing can effectively reduce the metal vapor leaking out
of the arc chamber. The difficulty caused by metal contamination on
the wafers in applying the plasma flood is therefore significant
reduced.
To sufficiently neutralize the ion beam, more plasma flood current
needs to be extracted out of the arc chamber, which requires one or
multiple extraction apertures with large total area. However, large
extraction area causes more bleeding gas leaking into the
neutralizing region and increases the vacuum pressure in the
region. It is not desirable for the vacuum pressure near the wafer
higher that 1E-5 torr. The vacuum pressure in the neutralizing
region is proportional to the extraction aperture area and the gas
pressure inside the arc chamber. Since the arc chamber pressure is
reduced by a factor of 4 comparing to a conventional source as we
have discussed above, the extraction aperture area can be increased
by a factor of 4 to keep the vacuum pressure unchanged. Larger
extraction area can provide more electrons from the source and thus
increase beam neutralization efficiency.
Although the present invention has been described in terms of the
presently preferred embodiment, it is to be understood that such
disclosure is not to be interpreted as limiting. Various
alternations and modifications will no doubt become apparent to
those skilled in the art after reading the above disclosure.
Accordingly, it is intended that the appended claims be interpreted
as covering all alternations and modifications as fall within the
true spirit and scope of the invention.
* * * * *